1. Field of the Invention
The present invention relates generally to equipment and methods for cementing liners in a wellbore. More particularly, this invention relates to a landing collar with a pressure indicator/bypass for use within a two plug cementing system.
2. Description of the Background
Liners have long been used in oil and gas recovery operations for many purposes, including completion flexibility, well control, reducing initial casing costs, repair purposes, and well deepening. In hydrocarbon recovery operations, a liner generally refers to a section or string of tubing, casing, or other similar oilfield tubular that is secured downhole within a larger diameter downhole tubular. Liners normally do not extend to the surface, and are sometimes referred to as a short string. A short string liner is typically fixed within a larger diameter casing and is structurally interconnected with the casing by a slip assembly. The short string liner may extend downward from the casing into an open hole region below the bottom of the casing.
It is often desirable to cement the liner into place by positioning cement into the annulus directly surrounding the liner. For this purpose, pump down plugs may be used both in front of and behind a column of cement that is pumped through the liner for circulation to this annulus about the liner. The pump down plugs isolate the cement column from other wellbore fluids, such as displacing fluids. The plugs also perform a wiping or cleaning function. In this way, the cement is ideally positioned directly within the annulus surrounding the liner, and the interior of the liner is preferably substantially free of cement.
In a typical short string liner system, the pump down plug engages a liner wiper plug which may be pre-positioned within the liner. The pump down plug and liner wiper plug are forced downwardly simultaneously within the liner string to displace the cement therefrom and to wipe the inner walls of the liner. One prior art system promoted by Baker Service Tools is the CF-Four Plug System, Product No. 269-25. Further details regarding prior art systems are disclosed in U.S. Pat. Nos. 4,842,069 and 5,052,488.
Several problems arise from the use of the Four Plug System that may result in a poor or failed cement job, or that may relate to the inability of the operator to distinguish at the surface whether the cement job is satisfactory. When problems occur or when it cannot be determined at the surface that the cementing job was successful, any apparently necessary repair may be costly and time consuming, depending on the type of apparent failure.
Even if the downhole components perform correctly, it is often difficult to accurately discern at the surface (1) when each liner wiper plug has been engaged by a respective pump down plug, (2) when the liner wiper plug is sheared from its position, and (3) when the pump down plug/liner wiper plug combination is set in its final position at the lower end of the liner to perform its designated function. Accordingly, the well operator may not know whether to advise performing a corrective squeeze job to squeeze additional cement behind the liner and, if so, how much cement preferably will be used for the squeeze job.
Other problems with prior art cementing operations may also arise. For various reasons, a liner wiper plug may shear prior to the time it was intended to shear, thereby causing a failed cement job. Multiple liner wiper plugs that are intended to drop sequentially may drop simultaneously. In some cases, one or more liner wiper plugs may not shear as intended. Because the cement column is typically already moving downwardly toward or through the liner when such problems occur, the cement may set up in a manner that makes repair difficult. Such problems may be accentuated if the hole tends to be more horizontal.
When two or more pump down plugs are used in a Four Plug System, each plug is sized differently to properly engage the correct one of the two downhole liner wiper plugs. If the pump down plugs are inadvertently dropped in the wrong order, then the operation fails.
Another problem arises if it is necessary to drill out the cementing shoe. The pump down plug or liner wiper plug (or combination thereof) may rotate with the drill bit to make drilling difficult.
A general problem is that the use of one or more pump down plugs with one or more liner wiper plugs requires fairly complex equipment that, for various reasons, lends itself to a greater likelihood of job failure.
There remains a need for less complex liner cementing equipment that offers more dependable operation at reduced levels of capital investment. Those skilled in the art have long sought and will appreciate that the present invention provides solutions to these and other problems. The disadvantages of the prior art are overcome by the present invention, and an improved cementing system is hereinafter disclosed for reliably setting a liner within the wellbore. Another aspect of the invention is an improved method of setting a liner in a wellbore, and also an improved landing collar for operating with plugs during the cementing operation.